It is required to further enlarge the capacity in a backbone transmission system as the amount of information (traffic) in the Internet increases. A coherent optical transmission technology has drawn attention as one of technologies for high capacity. In the coherent optical transmission technology, an AC (Alternating Current) signal component is received which is amplified by mixing signal light and LO (Local Oscillator) light in a coherent optical receiver device. At that time, the larger the optical output of the local oscillator (LO) light becomes, the larger amplifying operation acts on the signal light. Therefore, the receiving characteristics with the high S/N (Signal/Noise) ratio can be obtained by inputting the high-power local oscillator (LO) light compared with the signal light.
An example of such coherent optical receiver device is described in patent literature 1. The coherent optical receiver device described in patent literature 1 includes an attenuator, an optical coupler, a receiver, and a processor.
In the coherent optical receiving system, the intensity noise of the input signal and the output of the beat signal between the input signal and the local oscillator (LO) light are different in the reduction rate with attenuation of the input signal. That is to say, attenuating the power of the input signal causes the intensity noise of the input signal to drop at a faster rate than the beat signal.
For this reason, in the coherent optical receiver device in patent literature 1, it is said that the S/N ratio of the beat signal is improved by attenuating an input signal before the input signal is combined with a local oscillator (LO) signal. Furthermore, it is said that the attenuation of the input signal can be adjusted in response to real-time measurements of the S/N ratio of the beat signal by providing a feedback loop between the processor and an adjustable attenuator.
Patent literature 2 discloses a coherent optical receiver which includes an intensity adjusting means for adjusting intensity of input signal light, a converting means for converting an analog signal into a digital signal, a storage means for storing an amplitude value, and a control means for controlling the intensity adjusting means. The converting means converts the analog signal obtained by the photoelectric conversion of the combined light which is obtained by combining the signal light and the local oscillator light into the digital signal. The storage means stores a first amplitude value of the analog signal which is obtained using an input signal light with no waveform distortion as an input signal light with the local oscillator light being turned off. The control means controls the intensity adjustment value of the intensity adjusting means so that a second amplitude value of the analog signal, which is obtained using an input signal light in operation as an input signal light with the local oscillator light being turned off, will become equal to the first amplitude value. By employing such configuration, it is said that excellent reception characteristics can be achieved even if input signal light is suffering various waveform distortions.
Patent literature 1: Japanese Patent Application Laid-Open Publication No. 2001-249053 (paragraphs [0007]-[0016])
Patent literature 2: Japanese Patent Application Laid-Open Publication No. 2009-212994 (paragraphs [0017] and [0018])